1. Field of the Invention
This invention relates to a video duplication control system for playing back a video signal recorded on a recording medium, and limiting or preventing the reproduced video signal from being played back and recorded on another recording medium. It also relates to a video playback device, video recording device and video recording medium on which a video signal is recorded which use this video duplication control system.
2. Description of the Related Art
VTR (Video Tape recording devices) are a commonplace feature of modern-day life, and many kinds of software exist which can be played back on a VTR. Digital VTR or DVD (Digital Video Disks) playback devices are now a reality, and provide videos and sound of exceptionally high quality.
There is however a problem in that this great abundance of software can be copied without restriction, and several methods have already been proposed to prohibit this.
For example, for a VTR which outputs an analog video signal, one method to prevent copying uses a difference in the AGC (Automatic Gain Control) system, or in the APC (Automatic Phase Control) system, of the VTR recording device and of a monitor receiver which displays the video.
When the VTR employs AGC using a pseudo sync signal inserted in the video signal, a monitor receiver employs AGC which does not use this pseudo sync signal. In this anti-duplication method using a difference in the type of AGC, a very high level pseudo sync signal is inserted and output in the video signal supplied from the playback VTR to the recording VTR as an AGC sync signal.
When the VTR employs APC using the phase of a color burst in the video signal, the monitor receiver uses a different type of APC. In an anti-duplication method using a difference in the type of APC, the phase of the color burst in the video signal supplied from the playback VTR to the recording VTR is partially reversed.
The monitor receiver which receives the analog video signal from the playback VTR plays back the signal correctly without being affected by the high level pseudo sync signal in AGC or the partial phase reversal of the color burst signal in APC.
On the other hand when a VTR records, on a recording medium, the analog video signal into which pseudo sync signals have been inserted or the analog video signal which has been subjected to color burst phase reversal in the playback VTR as described hereabove, proper gain control or phase control based on the input signal cannot be performed, and so the video signal is not correctly recorded. Even if this signal is played back, therefore, normal picture and sound cannot be obtained.
In the case of a digitized video signal, e.g. in a digital VTR, an anti-duplication code or an anti-duplication control signal comprising, for example, a duplication ranking control code, is added as digital data to the video signal and recorded on the recording medium, so as to prevent or control duplication of the video.
In this case, the playback digital VTR reads the video signal which has been recorded on the recording medium, audio signal and anti-duplication control signal, and supplies them as digital or analog data to a recording digital VTR.
In the digital VTR being used as a recording device, the anti-duplication control signal is extracted from the supplied playback signal, and recording of the playback signal is then controlled based on the anti-duplication control signal. For example, when the anti-duplication control signal comprises an anti-duplication code, the recording digital VTR does not perform recording.
Alternatively, when the anti-duplication control signal comprises a copy ranking control code, recording is controlled by this ranking control code. For example, when the copy ranking code limits duplication to one copy, the digital VTR used for recording adds this anti-duplication code before recording the video signal and audio signal on the recording medium as digital data. It is thereafter impossible to duplicate the video signal from the copy.
Hence, in the case of a digital connection, that is when a video signal, an audio signal and anti-duplication control signal are supplied as digital signals to the digital VTR used as a recording device, anti-duplication control is performed on the recording side using the anti-duplication control signal by supplying this signal to the recording digital VTR as digital data.
However, in the case of an analog connection where the video signal and audio signal are supplied as analog signals, the anti-duplication control signal is lost when the signal supplied to the recording device is D/A converted. Hence, in the case of an analog connection, an anti-duplication control signal must be added to the D/A converted video or sound signal, and this causes deterioration of the video signal and audio signal.
It is, therefore, difficult to add an anti-duplication control signal and to extract it in the recorder for the purpose of anti-duplication control, without causing deterioration of the D/A converted video signal or audio signal.
Conventionally, therefore, duplication was prevented by an anti-duplication method using a difference in the AGC, or a difference in APC characteristics, between the VTR and the monitor receiver as described above.
However, in some cases, when duplication is prevented using the aforesaid difference in the AGC or a difference in APC characteristics between the VTR and the monitor receiver, depending on the type of AGC or APC characteristics on the recording side, the video signal may nevertheless be correctly recorded. In this case, it might happen that duplication cannot be prevented, or that the reproduced video on the monitor receiver is distorted. Further, it was troublesome to change over the anti-duplication method depending on whether there was an analog connection or a digital connection.
The inventors have already proposed a duplication prevention method in which a spectrally spread anti-duplication control signal is superimposed on a video signal (U.S. patent application Ser. No. 08175510). This method may be used for both digital connections and analog connections, and it causes no deterioration of the video or sound which is played back.
According to this method, when the original recording medium is fabricated, a PN (Pseudorandom Noise) sequence code (referred to hereafter as PN code) is generated with a sufficiently fast period and spectrally spread by applying it to the anti-duplication control signal. In this way, a narrow bandwidth, high level anti-duplication control signal is converted to a wideband, low level signal which does not affect the video signal or sound signal. This spectrally spread anti-duplication control signal is then superimposed on the video signal supplied to the recording medium and recorded.
On the recording side, a PN code is generated with the same timing and phase as the PN code used for spectral spread in the playback device. This generated PN code is applied to the video signal on which the anti-duplication control signal is superimposed so as to extract the original anti-duplication control signal, i.e. so as to perform reverse spectral spread. Anti-duplication is then controlled based on the anti-duplication control signal extracted by reverse spectral spread.
In this way, the anti-duplication control signal is spectrally spread and superimposed on the video signal as a wideband, low level signal in the playback device. It is therefore difficult for a person wishing to illegally duplicate the video signal, to remove the anti-duplication control signal which is superimposed on it.
However, it is possible for a person aiming to prevent illegal duplication to detect the superimposed anti-duplication control signal by reverse spectral spread and use that control. This anti-duplication control signal is therefore supplied to the recording device together with the video signal. On the recording side, the anti-duplication control signal is detected, and duplication is precisely controlled according to the detected anti-duplication control signal.
According to this method, as described hereabove, the spectrally spread anti-duplication control signal is superimposed as a wideband, low level signal on the video signal, but it must be superimposed at a lower S/N ratio than that of the video signal in order for the control signal not to cause deterioration of the video signal.
To superpose the spectrally spread anti-duplication control signal at a lower S/N ratio than that of the video signal, and to be able to detect the anti-duplication control signal which has been superimposed on the video signal in the recording device, the number of the PN code (PN code length) required to spectrally expand a 1 bit anti-duplication control signal must be sufficiently large. The PN code length per bit of the anti-duplication control signal may also be expressed as an spread gain (spread factor) which is the ratio (T/TC) of a time width T per bit of the anti-duplication control signal and a time width TC of one part (one chip) of the PN code. As described hereabove, this spread gain is found from the S/N ratio of the information signal on which the anti-duplication control signal is superimposed, in this case the S/N ratio of the video signal.
For example, when the S/N ratio of the video signal on which the anti-duplication control signal is superimposed is 50 dB, the anti-duplication control signal which is spectrally spread and superimposed on the video signal must be superimposed at a lower level than 50 dB, which is the S/N ratio of the video signal. Also, in order to detect the anti-duplication control signal superimposed on the video signal, its S/N ratio must be sufficient for the spectrally spread signal to be fully demodulated. If this S/N ratio is 10 dB, a spread gain of 60 dB (S/N ratio of 50 dB for video signal)+(S/N ratio of 10 dB necessary for detection) is required. In this case, the PN code length per bit of the anti-duplication control signal is 1 million.
The method used in the recording device to detect the PN code superimposed on the video signal uses a multifilter or a sliding correlation. In the former case, detection of the PN code is rapid, but only a short code length can be detected. At present, this code length is of the order of 256, and when the PN code length is 1 million per bit of the anti-duplication control signal, it cannot be detected. In the latter case, PN codes of long length can be detected but the detection takes time. It can thus be appreciated that a considerable time is required to detect a PN code having a length of 1 million.
In the recording device, if the anti-duplication control signal superimposed on the video signal can be extracted rapidly, the duplication control may be performed more effectively and efficiently by superimposing a spectrally spread anti-duplication control signal on the video signal.
This invention therefore aims to provide a video anti-duplication method, a video anti-duplication device and a video recording medium to permit additional information to be superimposed on a video signal so as to control duplication of the signal without causing its deterioration, and to control such duplication by precisely and rapidly extracting this additional information.
This invention provides a video transmitting method for superimposing a spectrally spread anti-duplication control signal on an analog video signal and transmitting the result, wherein:
a spread code for performing spread is generated in synchronism with a sync signal in the video signal, and
the anti-duplication control signal is spectrally spread using the spread code for performing spread.
In the video transmitting method according to this invention:
an analog video signal is received on which an anti-duplication control signal, which has been spectrally spread by a spread code synchronized with the video sync signal, is superimposed,
a spread code for performing reverse spread is generated in synchronism with the sync signal in the received video signal,
reverse spectral spread is performed using the spread code for performing reverse spread, and the anti-duplication control signal is extracted from the video signal.
In the video transmitting method according to this invention, a spread code is generated, for example, in synchronism with a vertical sync signal in the video signal on both the video signal output side and the video signal receiving side.
By using a sync signal in the video signal in this way, the spread code used for performing spectral spread on the output side can rapidly be synchronized with the spread code used for performing reverse spectral spread on the receiving side.